It is desirable for many different applications to monitor the temperature and the changes in the temperature of a medium. In particular, rapid monitoring of such changes is necessary and even required for many applications. For example, in engine environment monitoring and biomedical events monitoring, a response time of less than 5 seconds, or preferably less than 1 second is desirable. Applications requiring monitoring of radiation, power, heat and mass flow, charge and momentum flow, and phase transformation also demand rapid response times. Faster response times are, in general, the preferred performance even in applications that currently use devices that offer very slow response. For example, in ultra-precision manufacturing, temperature control is by far one of the most convenient methods of objective control structure. In machining at high speeds, the temperature of the tool or the substrate is a critical indicator of manufacturing efficacy; similarly welding, casting, milling, electrodischarge machining, chemical or laser etching of screens and stencils, bonding of dissimilar materials, lathe motor winding temperature, and related manufacturing equipment and processes are all thermally intensive, and the rapid sensing and control of temperature is critical to the end product quality. The response time of the thermal sensor determines the efficacy and the effectiveness of temperature control equipment for many applications including the monitoring of coolant and lubricant temperature before, during, and after an engine or motor operation, medical applications, icing of wings, phase transformations caused by physical or chemical effects, composition transformations caused by physical or chemical effects, the monitoring of pollution prevention units, exhausts, heaters, ovens, household apparatus, laboratory and industrial instruments, furnaces, and finally fire/heat detection and prevention systems.
The temperature of a medium is commonly monitored over a range using devices based on thermocouples, RTDs or thermistors. Thermocouples, by far the most common technique, are unsatisfactory in many applications as their response time is slow and often in the range of 30 to 500 seconds. RTDs are faster, however they are also unsatisfactory for applications that require rapid monitoring because their response time is 20 to 50 seconds even at higher temperatures. Of the known devices, thermistors are the best in their response times, but they are still limited to response times in the range of 5 to 25 seconds.
Thermistors are thermally sensitive resistors used in a variety of applications, including temperature measurement A thermistor is a piece of semiconductor made from metal oxides, pressed into a small bead, disk, wafer, or other shape, sintered at high temperatures, and finally coated with epoxy or glass. The resulting device exhibits an electrical resistance that varies with temperature. The two types of thermistors include: negative temperature coefficient (NTC) thermistors, whose resistance decreases with increasing temperature, and positive temperature coefficient (PTC) thermistors, whose resistance increases with increasing temperature. NTC thermistors are much more commonly used than PTC thermistors, especially for temperature measurement applications.
A main advantage of thermistors for temperature measurement is their high sensitivity. For example, a thermistor can have a sensitivity that is 10 or more fold higher than platinum-based RID which itself is about 3 to 10 fold more sensitive than thermocouples. The physically small size of the thermistor bead can also help yield a very fast response to temperature changes.
Another advantage of the thermistor is its relatively high resistance. Thermistors are available with base resistances (at 25xc2x0 C.) ranging from hundreds to millions of ohms. This high resistance diminishes the effect of inherent resistances in the lead wires, which can cause significant errors with low resistance devices such as RTDs. For example, while RTD measurements typically require 3-wire or 4-wire connections to reduce errors caused by lead wire resistances, 2-wire connections to thermistors are usually adequate. The major tradeoff for the high resistance and sensitivity of the thermistor is its highly nonlinear output and relatively limited operating range.
One drawback of thermistors, however, is their use over limited temperature ranges. Thermistors have been used primarily for high-resolution measurements over limited temperature ranges, and one example of such an application is medical thermometry.
Another drawback to the use of thermistors is that, because of their small size and high resistance, they are prone to self-heating errors. When current is passed through the thermistor, power dissipated by the thermistor, equal to I2R, will heat the thermistor. Manufacturers typically specify this as the dissipation constant, which is the power required to heat the thermistor 1xc2x0 C. from ambient temperature (mW/C.). The dissipation constant depends heavily on how easily heat is transferred away from the thermistor, so the dissipation constant may be specified for different media. This phenomenon is the basis of application of thermistor devices for monitoring of power, heat and mass flow, of charge and momentum flow, and of phase transformation. Nevertheless, a stable and reproducible dissipation constant is required in various applications; a requirement which state of the art thermistors usually fail to offer.
In summary, the slow response time, limited temperature range, the high thermal mass, the self-heating errors are the most important limitations of thermistors. This invention teaches a method of overcoming these limitations. Although this invention describes NTC thermistors, it would be obvious to those skilled in the art that the rationale and method discussed applies to practice of PTC thermistors as well. Furthermore, the rationale and method described in detail later also offers practical insights for the design and practice of superior RTDs and thermocouples as well. The teachings can be used to develop such devices that are superior in response characteristics, sensitivity, resistivity, stability, miniaturization, thermal mass, sintering temperature, electrode costs, and sintering time. Finally, while it is conventional to use thermistor""s resistance measurement for temperature monitoring, this invention""s teachings can also be easily extended to any electrical property of a thermal sensor, including but not limited to capacitance, inductance, impedance, conductance, admittance, and loss factor.
Briefly stated, the present invention involves a method of reducing the sintering temperature of a device by providing nanostructured powders of the active material of the device. The device is prepared from the nanostructured powders, and sintered at a temperature that is at least 100xc2x0 C. lower than the sintering temperature necessary for a device prepared from micron-sized powders.